So-called power line carrier local area networks have been designed according to the prior art and have the acknowledged advantage of reducing wiring costs and interference with everyday work during installation. However, the prior art devices have major disadvantages; namely, in order to achieve relatively long time error free transmission they are only able to achieve low data rates. Such prior art systems have utilized carrier frequencies in the range of 90 to 400 KHz. Major problems in utilizing an alternating current power line transmission channel or any other channel subject to electromagnetic interference are noise produced by motors, light dimmers, arc welders and the like. Specific problems of AC power line transmission channels are large changes from approximately one ohm to infinity in line impedance due to changing load conditions; average low impedance of the line in the order of 10 ohms; and effective carrier signals in the single digit voltage range. Such systems have utilized error detection and correction and redundant transmission in attempts to overcome these problems.
However, one of the fastest systems according to the prior art only produces an effective transmission rate of 240 baud at half duplex.
Thus, a leading supplier in the field has acknowledged "Unfortunately, an inherent limitation of PLC (power line carrier) today is the relatively slow transmission rate. Moreover, the AC power lines can support only one channel of simultaneous communication, although with multiple carriers this can be increased to four channels. Because of its low throughput, PLC cannot be used yet for centralized automation systems in large buildings because it cannot handle the high volume of data traffic between the computer and remote sites."
It is therefore highly desirable to overcome the problems of AC transmission lines as data transmission channels and to achieve local area networks utilizing AC transmission lines as the data transmission channels which provide relatively high speed, at least 1200 baud, full duplex, transparent, data transmission. It is also highly desirable to provide a system that provides for virtually error free transmission.
It is also desirable to achieve such local area networks utilizing other transmission channels in noisy environments, such as wire pairs.